Clarify AgentTask documentation

[bigboateng]

Summary

• position AgentTask as one bounded coding-agent judgment inside code workflows
• document schema, explicit-context, host-capability, and fixture-testing boundaries
• keep root and language README examples tied to tested workflow sources

Verification

• npm run format:check
• npm run test:release
• npm test && npm run build (sdk/typescript)
• python3 -m unittest discover -s sdk/python -p 'test_*.py'\n- go test ./...\n- cargo test --manifest-path sdk/rust/Cargo.toml\n- core examples plus examples/library/test-all.sh (40 workflows)